Landsat satellites have been collecting images of the Earth's surface for more than thirty years. NASA launched the first Landsat satellite in 1972, and the most recent one, Landsat 7, in 1999. Instruments onboard the satellites have acquired millions of images of the Earth. These images provide a unique resource for people who work in agriculture, geology, forestry, regional planning, education, mapping, and global change research.

.: Landsat 7
The Orbit of Landsat 7 is repetitive, circular, Sun-synchronous, and near polar at a nominal altitude of 705 km (438 miles) at the Equator. The spacecraft crosses the Equator from north to south on a descending orbital node from between 10:00 AM and 10:15 AM on each pass.

Circling the Earth at 7.5 km/sec, each orbit takes nearly 99 minutes. The spacecraft completes just over 14 orbits per day, covering the entire Earth between 81 degrees north and south latitude every 16 days. Figure 5.1 illustrates Landsat's orbit characteristics.

Landsat 7 and Terra were launched and injected into identical 705 kilometer, sun-synchronous orbits in 1999. This same day orbit configuration will space the satellites ideally 15 minutes apart (i.e. equatorial crossing times of 10:00 to 10:15 AM for Landsat 7 and 10:30 for Terra). A multispectral data set having both high (30 meter) and medium to coarse (250 to 1000 meter) spatial resolution will thus be acquired on a global basis repetitively and under nearly identical atmospheric and plant physiological conditions.